1. Field of the Invention
The present invention relates to low dielectric constant silica films and to improved processes for producing the same on substrates suitable for use in the production of integrated circuits.
2. Description of the Prior Art
As feature sizes in integrated circuits (ICs) approach 0.18 microns and below, it is believed that electrical insulation layers having a dielectric constantxe2x89xa62.5 will be required for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications.
Nanoporous Silica Films
One material with a low dielectric constant (xe2x80x9ckxe2x80x9d) is nanoporous silica, which can be prepared with relatively low dielectric constants, by the incorporation of air, with a k of 1, in the form of nanometer-scale pores. Nanoporous silica is attractive because it employs similar precursors, including organic-substituted silanes, e.g., tetramethoxysilane (xe2x80x9cTMOSxe2x80x9d) and/or tetraethoxysilane (xe2x80x9cTEOSxe2x80x9d), as are used for the currently employed spin-on-glasses (xe2x80x9cSOGxe2x80x9d) and chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) of silica (SiO2). Nanoporous silica is also attractive because it is possible to control the porosity, and hence the density, material strength and dielectric constant of the resulting film material. In addition to a low k, nanoporous silica offers other advantages including: 1) thermal stability to 900xc2x0 C., 2) substantially small pore size, i.e, at least an order of magnitude smaller in scale than the microelectronic features of the integrated circuit), 3) as noted above, preparation from materials such as silica and TEOS that are widely used in semiconductors, 4) the ability to xe2x80x9ctunexe2x80x9d the dielectric constant of nanoporous silica over a wide range, and 5) deposition of a nanoporous film can be achieved using tools similar to those employed for conventional SOG processing.
One difficulty associated with nanoporous silica films is the presence of polarizable functional groups on internal pore surfaces. Pore surface functional group""s present in previously available nanoporous films include silanol (SiOH), siloxane (SiOSi), alkoxy (SiOR), where R is an organic species such as, but not limited to, a methyl, ethyl, isopropyl, or phenyl groups, or an alkylsilane (SiR), where R is as defined previously. In particular, silanol groups are highly polarizable and hygroscopic. Since nanoporous silica has a relatively large (internal) surface area associated with its porous structure, the contribution of the highly polarizable silanol groups results in higher than desired dielectric constant values. Adsorption of environmental water by the silanol groups can potentially raise the dielectric constant of such materials even further.
Even if the dielectric film is outgassed by heating before subsequent processing, the presence of the polar silanols can contribute negatively to the dielectric constant and dielectric loss. Previously employed methods for overcoming this limitation and rendering the internal pore surfaces of nanoporous silica less polarizable and less hydrophilic include reacting the internal surface silanols with surface modifying agents, also referred to in the art as silylation agents or capping agents. Such capping agents include, e.g., chlorosilanes or disilazanes.
In one previously employed method of capping silanol groups on pore surfaces, an organic reagent such as hexamethyldisilazane (HMDZ) is introduced into the pores of the film and allowed to react with the surface silanol groups to cap the silanols by forming trimethylsilyl groups. These silylated surface groups are significantly less polarizable than the original silanols, and render the pore surfaces of the film hydrophobic. One disadvantage in the use of trimethylsilyl groups is that they are not very thermally stable and may out-gas during processing of the IC and cause via poisoning.
Another critical parameter of a nanoporous silica film is its mechanical strength. Generally the mechanical strength of a material decreases in proportion to any decrease in density in that material. For a nanoporous film to be useful as a dielectric film in IC devices, it is important that the combination of mechanical strength and low dielectric constant be optimized. For a given dielectric constant (which is proportional to refractive index and density), the density is fixed, at least for a specific chemical composition. With fixed density, the strength of the nanoporous silica is maximized by having the greatest fraction of solid within the skeleton of the film rather than as appended chemical groups on the surfaces of the nanometer-scale pores. Thus, in another drawback, reagents such as HMDZ introduce a significant additional mass, in the form of trimethylsilyl groups, to the pore surfaces. The disproportionate mass of the trimethylsilyl groups is not available to contribute to mechanical strength, but it does raise the density of the film and therefore is an obstacle to achieving the lowest possible k.
Thus, in view of the need for rapid competitive advances in the art of microprocessor, i.e., IC fabrication, there remains a constant need in the art to improve upon previous methods and materials. In particular, there is a need to provide nanoporous silica films with hydrophobic pore surfaces, while minimizing mass introduced by hydrophobic surface modification agents at the nanometer-scale pore surfaces. The successful solution of this problem will provide greater material film strength for a given desired dielectric constant.
Surprisingly, the methods of the present invention are able to solve these and other problems in the art by providing multifunctional surface modification agents that are able to significantly reduce the additional appended mass and density added to silica dielectric films when capping free silanols, relative to previously employed methods and agents or reagents.
Accordingly, the invention provides novel processes for forming dielectric films or coatings on a desired substrate by the steps of (a) preparing a silica dielectric film on a substrate by any suitable methods, e.g., by any art known methods that produce a silica films in need of the surface modification provided by the present invention; and (b) reacting the film of (a) with a multifunctional surface modification agent under conditions, and for a time period, that is effective to silylate a substantial proportion of any free silanol groups on the film surfaces, so as to render the treated surface less polarizable and non-hygroscopic.
The silica dielectric film to be treated may be non-porous, but is preferably a nanoporous silica film prepared on the substrate immediately prior to the time of treatment, or may be previously prepared and stored or obtained from another source. It should also be mentioned that the silica dielectric films to be treated by the novel processes of the invention are optionally aged before or after conducting the surface modification treatment as described herein, but preferably, the film is aged prior to surface modification.
The inventive processes of the invention may be conducted using either a vapor phase or a liquid phase surface modification agent, as desired. Further, the processes are optionally conducted in the presence of a solvent or co-solvent, and it should be appreciated that when the surface modification is to be conducted in the liquid phase, the solvent or co-solvent will dissolve the surface modification agent without significantly dissolving the film to be treated. Any suitable material may be employed as a solvent or co-solvent, but preferably, such solvent or co-solvent is selected from the group consisting of ethers, esters, ketones, glycol ethers, chlorinated solvents, low viscosity siloxanes and suitable combinations thereof. In one alternative embodiment, the solvent or co-solvent is not a ketone.
While the silica film to be treated need not be porous, preferably, the film to be treated is a nanoporous dielectric film having a pore structure with hydrophilic pore surfaces, and the surface modification process is conducted for a period of time sufficient for the surface modification agent to penetrate the pore structure and to produce a treated nanoporous silica film having a dielectric constant of about 3 or less. Preferably, the surface modification reaction is conducted for a time period ranging from about 10 seconds to about 1 hour and at a temperature ranging from about 10xc2x0 C. to about 100xc2x0 C.
Preferably the film to be treated is on a substrate, e.g., a wafer suitable for production of an integrated circuit.
The invention also provides for dielectric films and an integrated circuit or circuits that include at least one dielectric film produced by the processes of the invention. Preferably, dielectric silica-based films produced by the inventive processes reveal no significant silanol absorbance in the wavelengths ranging from about 3000 to about 3700 cmxe2x88x921 under Fourier transform infrared spectroscopy.